1. Field of the Invention
The present invention relates to a technique for transferring a toner image formed on an image bearing member to a recording material by using the electrophotographic process, and also to a technique for separating a thin recording material from a transfer belt conveying the recording material, used in image forming apparatuses such as copying machines and laser beam printers.
2. Description of the Related Art
In recent years, there has been a demand for application of diverse types of recording materials to image forming apparatuses utilizing the electrophotographic process, such as copying machines and laser beam printers. For example, there has been a demand for application of a recording material having a low stiffness, such as thin paper, to image forming apparatuses. However, because of a low stiffness, the use of such thin paper requires a recording material conveyance system that can ensure high stability.
To meet such a demand, an applied technique electrostatistically attracts a recording material to the transfer belt conveying the recording material to allow it to pass through a transfer nip portion.
However, when such a technique is applied to a recording material having a low stiffness, a paper sheet sticks to the transfer belt at the leading end of a separation roller, where the recording material should be separated from the transfer belt, resulting in separation failure.
As a method for solving this problem, undulating the transfer belt at a separation position makes the recording material stiffer, improving the separation between the recording material and the transfer belt. More specifically, projections are uniformly formed on the surface of the separation roller that supports the transfer belt at the separation position (Japanese Patent Application Laid-Open No. 8-113408). However, when the thus-configured separation roller is used to form undulations on the transfer belt, the separation roller will locally apply large tension to the transfer belt on a constant basis. As a result, resistance unevenness due to local wear on the transfer belt causes unstable transferability.
Japanese Patent Application Laid-Open No. 5-119636 discusses a technique for deforming a sheet-like transfer drum while reducing wear due to deformation. With the technique in Japanese Patent Application Laid-Open No. 5-119636, there is provided a roller movable to a position where the sheet surface on the transfer drum is raised from the inner side and a position where it is not raised. While the recording material is not separated, the sheet surface is not raised. When the recording material is separated from the sheet surface, the sheet surface is raised.
When this configuration is applied to the transfer belt, a raising means for locally raising the transfer belt at the time of separation process is disposed on the upstream side of the separation roller. With an extremely thin recording material, conveying the recording material with the transfer belt locally raised deforms the transfer belt and recording material, making the recording material stiffer. This configuration allows even an extremely thin recording material to be separated from the transfer belt.
However, when the raising means locally raises the transfer belt, if the influence of deformation on the belt surface of the transfer belt reaches a transfer portion, the transfer nip portion becomes unstable possibly resulting in transfer failure.